The invention relates to dental ceramics and a method for the manufacture thereof.
Ceramics which are suitable as dental ceramics for use in dental preparations has to satisfy special requirements. They should have particularly little chemical solubility and a color that can be adjusted to the basic dental colors or color adjustment using coloring agents.
The German DIN EN ISO 6872 sets the upper limit for the chemical solubility of dental ceramics as 100 μg/cm2.
The color is characterized according to the CIE-L*a*b*-system of the Commision Internationale de l'Eclairage (CIE), Vienna, 1986. In this system, the following parameters are used:
L* the lightness perception
A* the red-green perception wherein a*>0 is a red and a*<0 is a green perception
B* the yellow-blue perception, wherein b*>0 is a yellow, and
b*<0 is a blue perception in each case of the retina of the human eye.
For fully ceramic artificial teeth silicate- or oxide-ceramics are suitable, wherein the oxide ceramics, in comparison for example with so-called press ceramics, cannot be brought simply into a desired shape. From ceramic powders first so-called green bodies are manufactured which are subsequently sintered. During sintering of the porous green bodies, the powder particles are densified to form a solid ceramic structure wherein however necessarily a volume shrinkage of up to 50% occurs. This sinter shrinkage must be taken into consideration during the manufacture of the ceramic tooth replacements.
DE 195 47 129 discloses a method for a shrinkage-free manufacture of oxide-ceramic sinter bodies of powder-like oxide ceramics such as zirconium dioxide or aluminum oxide, a powder-like inter-metallic compound and a silicon organic polymer. However, not every possible composition results in a ceramic structure which satisfies the above-mentioned requirements for dental ceramics. Generally, ceramics manufactured in this way are not white but greenish-yellow and the chemical solubility thereof is excessively high.
In accordance with J. R. Binder, S. Ray and E. Klose, “Schwindungsfreie Kermiken im SystemZr—Si—O” (Shrinkage-free Ceramics in the system Zr—Si—O) DKG manual, chapter 3, 6, 4.4, 2002, the manufacture of shrinkage-free ceramics in the system zirconium-silicon-oxide (Zr—Si—O) is based on the compensation of the shrinkage unavoidably occurring during sintering by means of the volume-increasing oxidation reaction of zirconium silicide (ZrySix). The mechanical properties of such reaction-sintered ceramics herein depend to a large extend on the Zr/Si ratio.
H. Salmang and H. Scholse, Keramik, Part 2. Keramische Werkstoffe (Ceramics materials)6th. Edition, 1983. Springer Verlag, Pages 168-169 discloses the use of an additive of 0.1 to 0.5 wt % magnesium oxide MgO as sintering aid in the manufacture of aluminum oxide (Al2O3).
The product information sheet “New Improved Zirconia Powder” of the company Tosoh, Japan, 1999 discloses that an addition of 0.25 wt % aluminum oxide Al2O3 added as sintering aid to zirconium dioxide reduces the sintering temperature by about 100° C.
It is now the object of the present invention to provide dental ceramics and a method for the manufacture thereof which do not have the limitations and disadvantages mentioned above. Particularly dental ceramics should be provided which permit a shrinkage-free manufacture and which have a low chemical solubility and also a color which facilitates a coloring to the basic dental colors or which permits coloring by coloring agents.